cppgc-js: Fix snapshot node merging

In Blink, WindowProxy may be referred from two diffrent JS wrapper
objects during page refresh (same site navigation reusing parts of the
DOM). In this intermediate state, the old frame state is not yet
reclaimed while the new state is already being added.

We would like to only merge nodes when there's a 1:1 relation between
C++ and JS objects. Unfortunately, WindowProxy breaks that assumption
in that the C++ object doesn't directly point to the wrapper. In
addition, merging this case is important as otherwise detachedness
would not be propagated to the Window object (JS wrapper) which is the
main user of detachedness.

The CL allows overriding merged nodes, picking a random merged state
during pageload while still resulting in the regular snapshot behavior
outside of reloading the same page.

The proper fix is addressing chromium:1218404 and only create merged
nodes when the back reference points to the same object.

Bug: chromium:1241610
Change-Id: Ie77b51a56ce90ef377124304bb025342a724c600
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3114139Reviewed-by: Anton Bikineev <bikineev@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#76453}

src/heap/cppgc-js/cpp-snapshot.cc

@@ -47,7 +47,13 @@ class EmbedderNode : public v8::EmbedderGraph::Node {
   void SetWrapperNode(v8::EmbedderGraph::Node* wrapper_node) {
     // An embedder node may only be merged with a single wrapper node, as
     // consumers of the graph may merge a node and its wrapper node.
-    DCHECK_NULL(wrapper_node_);
+    //
+    // TODO(chromium:1218404): Add a DCHECK() to avoid overriding an already
+    // set `wrapper_node_`. This can currently happen with global proxies that
+    // are rewired (and still kept alive) after reloading a page, see
+    // `CreateMergedNode`. We accept overriding the wrapper node in such cases,
+    // leading to a random merged node and separated nodes for all other
+    // proxies.
     wrapper_node_ = wrapper_node;
   }
   Node* WrapperNode() final { return wrapper_node_; }